Mail servers aren't real people or real businesses.
50 year old decentralized protocol has problems, so decentralized protocols can't possible work. Just like 50 year old computer can't play doom, so doom doesn't exist.
Search systems aren't real people or real businesses either. Never mind details like how you define and enforce "real" in a decentralized way across N legal systems with wildly varying ideas of what it means to be a "legitimate" business. There is no system or ledger you can query to find out if a given person is real or a business is legitimate. This is exceptionally unlikely to change on a useful timescale.
The basic problem with email is that it assumed good-faith participation from the parties involved. It was assumed that only legitimate actors would have the resources to participate and they would always be well-behaved. This, it turns out, was flawed on several counts. For one, it assumed legitimacy could be assured. For another, it assumed legitimate users would be well-behaved and would never abuse services for gain. For a third, it assumed account takeovers or other impersonation attacks wouldn't happen.
Every de-centralized system that aspires to not have email's problems needs to take them seriously.
The basic problem with email is that it assumed good-faith participation from the parties involved. It was assumed that only legitimate actors would have the resources to participate and they would always be well-behaved. This, it turns out, was flawed on several counts. For one, it assumed legitimacy could be assured. For another, it assumed legitimate users would be well-behaved and would never abuse services for gain. For a third, it assumed account takeovers or other impersonation attacks wouldn't happen.
Every de-centralized system that aspires to not have email's problems needs to take them seriously.